Banging gearbox

I would appreciate some help with an issue I have with the automatic gearbox in my 2002 BMW 540i with the ZF 5HP24 transmission. When coming to a stop, the gearbox bangs down into first gear. It can be controlled with a very, very gradual stop, but in normal driving it happens at every stop. The issue only occurs when the gearbox has warmed up.

I have a C110 code scanner and have noted that the problem starts when the gearbox reaches around 90°C and I have seen the gearbox reach 107°. The car has done 133,000 miles and, as far as I know, has never had a gearbox oil and filter change. I’ve checked the fluid level with the car running and it is fine. Every other aspect of the gearbox works well, including kickdown and manual selection mode.

One other point that may be relevant is that the car will drive at around 5mph on the flat in forward and reverse without pressing the accelerator pedal. I have never owned a car with an automatic gearbox before, so I’m not sure if this is normal. I’d also like to add that the gear selection between Park, Neutral, Reverse and Drive is fine when the car is stone cold, but when the car has warmed up a little bit the selector becomes stiff to move. The stiffness occurs before the temperature at which the transmission ‘thump’ occurs. I’ve greased the selector joints underneath the car but this hasn’t helped. R Darvill Answering your last point first, provided the engine idle speed is correct, it is quite normal for the vehicle to creep forward if left in gear and unchecked. As long as the engine is not labouring when pressing the brake to prevent this movement then there is nothing to worry about.

As the vehicle has covered 133,000 miles without any evidence of having had the gearbox oil changed, I would start by doing an oil change to see if any improvement is made. If there is no improvement, the problem may be due to a faulty speed sensor in the gearbox. There are two sensors that detect input and output speeds, and you should be able to check these using your C110 scanner to read off the live data. I would list the transmission temperature sensor as another possibility, but you have already checked this and ruled it out.

Another problem which can occur on the 5HP 24 gearbox is that the valve body can crack, releasing pressure from the seals. This normally only occurs when the ’box is warm and the valve body slightly expanded by the heat. I am not sure if this would affect the actual gearchange, but it is a possibility.